Evaluating the Role of Triborane(7) As Catalyst in the Pyrolysis of Tetraborane(10)

2013-09-19T00:00:00Z (GMT) by Baili Sun Michael L. McKee
The initial steps in the B<sub>4</sub>H<sub>10</sub> pyrolysis mechanism have been elucidated. The mechanism can be divided into three stages: initial formation of B<sub>4</sub>H<sub>8</sub>, production of volatile boranes with B<sub>3</sub>H<sub>7</sub> acting as a catalyst, and formation of nonvolatile products. The first step is B<sub>4</sub>H<sub>10</sub> decomposition to either B<sub>4</sub>H<sub>8</sub>/H<sub>2</sub> or B<sub>3</sub>H<sub>7</sub>/BH<sub>3</sub> where the free energy barrier for the first pathway is 5.6 kcal/mol higher (G4, 333 K) than the second pathway when transition state theory (TST) is used. When variation transition state theory (VTST) is used for formation of B<sub>3</sub>H<sub>7</sub>/BH<sub>3</sub>, the two pathways become very similar in free energy with the B<sub>4</sub>H<sub>8</sub>/H<sub>2</sub> pathway becoming favored at G4 by 1.0 kcal/mol at 333 K (33.1 versus 34.1 kcal/mol). The experimental activation energy for B<sub>4</sub>H<sub>10</sub> pyrolysis is about 10 kcal/mol lower than the calculated barrier for B<sub>4</sub>H<sub>10</sub> → B<sub>4</sub>H<sub>8</sub> + H<sub>2</sub>, which indicates that this reaction is not the rate-determining step. We suggest that the rate-determining step is B<sub>4</sub>H<sub>10</sub> + B<sub>3</sub>H<sub>7</sub> → B<sub>4</sub>H<sub>8</sub> + H<sub>2</sub> + B<sub>3</sub>H<sub>7</sub> where B<sub>3</sub>H<sub>7</sub> acts as a catalyst. The role of reactive boron hydrides such as B<sub>3</sub>H<sub>7</sub> and B<sub>4</sub>H<sub>8</sub> as catalysts in the build-up of larger boron hydrides may be more common than that previously considered.